Present invention relates generally to stepper motors and more particularly to stepper motor controllers.
Stepper motors are often utilized as mechanical positioning means. An example of equipment in which stepper motors are utilized in this manner are electronic disk drive units. Disk drive units have at least one magnetic record media rotated around a central pivot. One or more transducers are then positioned with respect to a plurality of parallel tracks on the record media. Stepper motors, along with linkage translating the rotational position of the stepper motor to a linear position along the record media surface, have been used for this purpose.
Positioning accuracy of the transducer along the record media surface is extremely important. The degree to which the positioning accuracy is achieved to select a given predetermined track on the record media surface is one of the factors that determines the density of recording that can occur on the record media. The positioning of the transducer with respect to a track on the record media surface must not only be accurate but repeatable. A move of the transducer to a particular track on the record media (disk platter), or a particular step of the stepper motor, must accurately repeat the position obtained in a prior move to that particular track. This is necessary to ensure the accurate writing and subsequent reading of data recorded on the record media surface. The positioning must be accurate no matter what the intervening positions or movements of the transducer have been. The transducer must get to the same accurate position if coming from a long distance along the record media surface or from the adjacent track. Further, the positioning must be accurate when the given track is approached from either side, i.e. approached with either an outer movement of the transducer or an inner movement of the transducer.
In many stepper motor positioning applications, the number of positions available exceeds the number of steps available in the stepper motor. In this case it is not sufficient just to specify the step number of the stepper motor to get to a particular position number (or track number). This is because more than one position exists for each step number of the stepper motor. Thus, the stepper motor must be sequenced through a number of steps in a particular order to move in a particular direction cycling through the set of steps of the stepper motor (or a partial set) in order to reach the desired final position (or track). Controllers to perform this sequencing function are well known in the art. Such controllers know which track (or position) of the record media at which the transducer is located. The controller then can accept a request to move to a new track (position). The controller then is able to calculate the number of steps in the requested move, the direction of move and sequence the stepper motor through the proper number and sequence of steps in order to reach the requested track (position).
It is known that with certain mechanical linkages or positioning systems that mechanically approaching a particular position (or step) from one direction may yield a slightly different position than approaching from the opposite direction. This is sometimes due to the mechanical viscosity and internal friction of the positioning system. With systems with such mechanical hysteresis, a vibration or a slight jarring of the positioning mechanism of the transducer can overcome this internal friction of the mechanical elements and eliminate the positioning differences.
With a disk drive unit having a transducer positioned with a stepper motor, it has been found that positioning differences exist from the direction of approach even after mechanical hysteresis is eliminated from the system. These positioning errors detract from the recording density otherwise achievable in the record media.
Other attempts have been made to solve this positioning problem. First, a track-following servo system has been utilized. These systems use a feedback from the recorded track to adjust the position of the transducer to follow that track. While this system solves the problem, it is only through the use of an expensive, sophisticated positioning system. A second solution to solve the positioning error problem is to simply overpower the hysteresis positioning problem by using a larger, more powerful stepper motor. This method is also successful in overcoming hysteresis but has other drawbacks. For example, a larger motor requires a larger power supply which in turn creates more heat and a greater heat differential. The greater heat differential creates differing thermal expansions of the mechanical components linking the transducer positioning system and also affects the positioning accuracy of the transducer.